How Substances are Transported and Removed 219 



which are very well illustrated in our figure 72. These sieve-tubes 

 accompany the ducts all through the plant from root-tips to 

 stem-tips and leaf-cells, as our generalized plant illustrates so 

 clearly (figure 54), thus forming a part of the same fibro-vascular 

 bundles. But sieve-tubes are more slender than ducts, and unlike 

 them have thin soft walls, and a continuous lining of protoplasm; 

 while the occasional cross partitions, thicker than the walls, are 

 perforated by openings in a way which has given these structures 

 their name (figure 54, C). The presence of this protoplasmic 

 lining in the sieve-tubes when diffusion alone does not require its 

 presence at all, suggests that it plays some part in helping to force 

 substances along the tubes, perhaps in a manner analogous to the 

 way in which the food is moved along the intestines of animals; 

 but no such action has been proven. Doubtless the movement is 

 aided materially by the swaying of branches in the wind, and, 

 when it is downwards, by gravitation; but these influences are 

 obviously both incidental and irregular, and diffusion is the only 

 motive force in translocation that we surely know. The reader, 

 therefore, must visualize this process as one of constant diffusion 

 along the sieve-tubes. It is not an onward movement of the 

 solution they contain, but a movement of the sugar and other dis- 

 solved substances through water that is standing still, a process 

 in great contrast with the onward rush of sugar-carrying sap in 

 the spring. The method of this diffusion, by the way, is illus- 

 trated diagrammatically in figure 6. 



The sieve-tubes, in which translocation of food principally 

 proceeds, lie in the inner bark of woody plants, down through 

 which, accordingly, all summer long, there is a constant move- 

 ment of food-substances towards the roots or other underground 

 parts devoted to winter storage. That this is really the path is 

 easily proven by experiment, such for instance as removing a 

 narrow ring of the bark, or constricting it by a metal ring. This 

 often happens by accident in Botanical Gardens where the en- 

 circling wires which support the labels are left too tight. In all 



